There has been considerable interest in the development of coating compositions with reduced amounts of volatile organic compounds (VOCs), particularly in light of regulations on VOC content. Ultraviolet (UV) curable coating compositions have emerged as one type of coating with reduced VOC content. Furthermore, many UV curable coating compositions offer the advantages of fast curing speed, high energy efficiency, and low capital investment and space requirements. These coatings are especially suitable for the wood/plastics coating (e.g., furniture), metal decorating, and paper printing industries.
The curing reaction of many UV curable coatings takes place at nearly ambient temperatures and, typically, no solvent or water evaporation is involved. There are two principle mechanisms for polymerizing UV coatings; free radical and cationic. In free radical initiated polymerization, free radicals are generated using either unimolecular photoinitiators (benziketals) or bimolecular photoinitiators (thioxanthones). Multi-functional acrylate esters are often used in this type of polymerization. Free radical polymerization, however, often exhibits oxygen inhibition of propagating radicals and acrylate monomers also represent odor and health hazard problems.
For cationic UV polymerization, typical initiators are salts which generate super acids or Bronsted acids under UV irradiation. Typical monomers/oligomers for the cationic coatings include, for example, vinyl ethers, propenyl ethers, and compounds having epoxide functional groups. These compounds can homopolymerize to form a coating.
Epoxide-containing compounds have fast cure response and the resulting cured coating is resistant to hydrolysis. In addition, aliphatic epoxide-containing compounds have excellent photostability. Epoxide-containing compounds, however, typically polymerize to form hard, brittle coatings. To increase the impact resistance, toughness, and flexibility of the cured coating, polyols have been added to the curable coating composition. The epoxide-containing compounds react with the polyols during the polymerization process.
Polyols act as reactive diluents in these curable coating compositions. Reactive diluents function as solvents to dilute viscous oligomers and reduce the overall viscosity of the curable coating composition. Reactive diluents also co-react with the monomer and oligomers to form integral films. Reactive diluents are typically compounds with relatively low molecular weights and multiple reactive functionalities. There is a need for the development of reactive diluents for use with epoxide-containing compounds to provide desired properties to films formed using UV curable coating compositions.